A class of fluorine-doped optical fibres has been tailored to fibre-to-the-premises (FTTP) applications, including fibre-to-the-home (FTTH) and fibre-to-the-building (FTTB) applications, which generally require low bending loss of optical signals transmitted through the fibres, also under stringent installation constraints that may impose tight bend radii, e.g., due to sharp cornering in buildings or compression of optical fibres. Optical transmission fibres designed for FTTP applications are in general required to meet the criteria of standardized recommendations that define single-mode fibres with enhanced bending performance, in particular the G.657 standard laid down by the International Telecommunications Union, ITU Telecommunication Sector (ITU-T).
Optical fibers having low bending loss may be obtained by fluorine-doping a region surrounding the fiber core, in order to suitably shape the refractive index profile.
A fluorine-doped single-mode optical fibre which is said to conform to the ITU-T G.652 specification standard and having an annular ring region of low relative refractive index of less than −3×10−3 is disclosed in EP 1 785 754. The described fibre is said to be preferably made by chemical a vapour deposition technique, specifically by Plasma Chemical Vapour Deposition (PCVD).
The incorporation of fluorine into a preform for optical fibre can be obtained by chemical vapour deposition (CVD) methods (such as modified CVD, MCVD; plasma CVD, PCVD; plasma enhanced CVD, PMCVD; outside vapour deposition, OVD; vapour axial deposition, VAD), typically employing fluorine precursors (e.g. SiF4, SF6, C2F4). Such methods require special plants and safety practice due to the presence of fluorine containing fluids.
A known alternative method to produce a fluorinated glass preform for use in the fabrication of a single-mode optical fibre is the so-called rod-in-tube (RIT) method, also known as “sleeving method”. For example, U.S. Pat. No. 4,846,867 concerns an RIT method for producing a primary glass preform by inserting a core rod in a glass tube made of quartz glass added with fluorine and collapsing the glass tube into the rod. In particular, U.S. Pat. No. 4,846,867 sets forth that during heating, the inner wall of the glass tube and the outer surface of the core rod are kept clean by gas phase etching with a halogen-containing compound. Subsequently, soot particles of SiO2 are deposited and then the preform having the layer of SiO2 particles is heated in an atmosphere comprising a fluorine-containing compound in a furnace.
As evidenced, for example, by U.S. Pat. No. 6,422,042, when a fluorine-doped silica tube is collapsed onto a germania-doped silica rod, the resultant interface between those two members can contain many seeds, and much of the resultant preform or blank produces unusable optical fibre. Seeds include defects such as bubbles and impurities that can produce attenuation in the resultant optical fibre.
In said U.S. Pat. No. 6,422,042, while the entire assembly is being heated, a centerline chlorine-containing gas is flowed into the first end of the tube and between the tube and the rod, and out of the second end of the tube. The tube collapse step can be performed in the same furnace in which the chlorine cleaning step occurs.
US 2008/0260339 discloses a method for making a depressed index cladding for the inner cladding of an optical fibre, wherein the fluorine doped inner cladding is made in two steps. A first step, using soot formation, produces the innermost portion of the inner cladding layer, preferably by vapour axial deposition (VAD), followed by a second step, using RIT, to complete the inner cladding layer. After assembly of the rod and tube the combination is fused in a furnace to produce the preform.